Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, i.e. a plasma, in a transparent tube. The character of the gas discharge critically depends on the frequency or modulation of the current. Typically, such lamps use a noble gas (argon, neon, krypton and xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury, sodium, and/or metal halides. In operation, the gas is ionized, and free electrons, accelerated by an electrical field in the tube, collide with gas and metal atoms. Some electrons circling around the gas and metal atoms are excited by these collisions, bringing them to a higher energy state. When the electron falls back to its original state, it emits a photon, resulting in a visible light or ultraviolet radiation. Ultraviolet radiation is converted to visible light by a fluorescent coating on the inside of the tube's surface for some lamp types. The fluorescent lamp is perhaps the best known gas-discharge lamp.
Gas-discharge lamps offer long life and high light efficiency, but are more complicated to manufacture, and they require electronics to provide the correct current flow through the gas.
A high-intensity discharge (HID) lamp is a type of electrical lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with both gas and metal salts. The gas facilitates the arc's initial strike. Once the arc is started, it heats and evaporates the metal salts forming a plasma, which greatly increases the intensity of light produced by the arc and reduces its power consumption. High-intensity discharge lamps are a type of arc lamp.
Compared with fluorescent and incandescent lamps, HID lamps have higher luminous efficacy since a greater proportion of their radiation is in visible light as opposed to heat. Their overall luminous efficacy is also much higher: they give a greater amount of light output per watt of electricity input.
Various different types of chemistry are used in the arc tubes of HID lamps depending on the desired characteristics of light intensity, correlated color temperature, color rendering index (CRI), energy efficiency, and lifespan. Varieties of HID lamp include: mercury vapor lamps, metal halide (MH) lamps, ceramic MH lamps, sodium vapor lamps, xenon short-arc lamps, and ultra-high performance (UHP).
Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs. The method used to initially strike the arc varies: mercury vapor lamps and some metal halide lamps are usually started using a third electrode near one of the main electrodes while other lamp styles are usually started using pulses of high voltage.
HID lamps are typically used when high levels of light over large areas are required, and when energy efficiency and/or light intensity are desired. These areas include gymnasiums, large public areas, warehouses, movie theaters, football stadiums, outdoor activity areas, roadways, parking lots, and pathways. More recently, HID lamps, especially metal halide, have been used in small retail and residential environments. HID lamps have made indoor gardening practical, particularly for plants that require a good deal of high intensity sunlight. They are also used to reproduce tropical intensity sunlight for indoor aquariums. Ultra-High Performance (UHP) HID lamps are used in LCD or DLP projection TV sets or projection displays.
Beginning in the early 1990s, HID lamps have been employed in motor vehicle headlamps. HID lamps are also used in high-performance bicycle headlamps as well as flashlights and other portable lights, because they produce a great amount of light per unit of power. As the HID lights use less than half the power of an equivalent tungsten-halogen light, a significantly smaller and lighter-weight power supply can be used. HID lamps have also become common on many aircraft as replacements for traditional landing and taxi lights.
Factors of wear come mostly from on/off cycles versus the total on time. The highest wear occurs when the HID burner is ignited while still hot and before the metallic salts have recrystallized. At the end of life, many types of high-intensity discharge lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage. As they heat up during operation, however, the internal gas pressure within the arc tube rises and a higher voltage is required to maintain the arc discharge. As a lamp gets older, the voltage necessary to maintain the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.